What class should this assassin be?

[Talionis]

I'm looking for a stealthy character that can impersonate other characters and preferably has a natural attack with poison that can easily be hidden. I'm not sure if I want a graft. I was thinking of a poison spike or stinger that comes out of the palm of his hand, so he could appear to be unarmed.

I was also looking at him possibly being associated with Spiders or Scorpions because of his stinger. Snakelord was a class I thought about, but I like the insects better than Snakes.

I was hoping to make the Kasami Gari his preferred weapon and have him be good at tripping and grappling. In actual combat.

Any thoughts on possible builds and classes that would work for this would be appreciated.

[HalfDragonCube]

Hmmm... Changeling Factotum?

You can change your appearance at will and you get decent SLAs after a while and a bunch of other cool things.

[HappyBlanket]

I'm not familiar with them, but there ought to be a few Arachnid themed templates. Here's a compiled list of templates (http://community.wizards.com/go/thread/view/75882/19869266/Master_Template_List), with thanks to google. Templates can probably get you that natural attack too. Changeling is a good race to take; as aforementioned, it can change it's physical appearance at-will. And Factotum is great for general rogueish archetypes.

Anyway, it sounds like a fun idea, so I wish I could help more :D

[Biffoniacus_Furiou]

Unarmed Swordsage, get Versatile Unarmed Strike to deal your choice of bludgeoning, piercing, or slashing damage with every attack. You can use Setting Sun maneuvers for tripping and throwing opponents. Maybe dip Monk to get Stunning Fist so you can take Rattlesnake Strike in Sandstorm. You could also use Poison Dusk Lizardfolk (MM3), which has a +1 LA but gets two claw attacks and has Poison Use as a racial trait. You can full attack with unarmed strikes and then make two secondary claw attacks, they'll be at -5 to hit but Multiattack reduces that to only a -2.

[Runestar]

Aranea with class lvs? They do lose their bite attack while in alternate form, so using it under disguise can be tricky.

[Midnight_v]

I don't know what level you're looking for but...
I agree the race should be changling, I suspect the class should be psychic warrior. psonic minor creation to whip out the most extreme poisons at will pretty much. I guess you could be an egoist or a wilder as well ...

[Telonius]

Cancer Mage (BoVD) has some abilities similar to what the OP is describing.

EDIT: Sure you don't want snakes? Yuan-ti might be a great race for it.